Page 32 - Control Theory in Biomedical Engineering

Modeling and control in physiology 19

Bradford Cannon (Cannon, 1929). Cannon combined two words from
Ancient Greek ὅμος (ho ´mos, “similar”)+ιστημι (hist emi, “standing
still”)/stasis (from στάσις) into a Modern Latin form (Davies, 2016). Can-
non wrote, “The constant conditions which are maintained in the body
might be termed equilibria. That word, however, has come to have exact
meaning as applied to relatively simple physico-chemical states, in closed
systems, where known forces are balanced. The coordinated physiological
processes which maintain most of the steady states in the organism are so
complex and so peculiar to living beings—involving, as they may, the brain
and nerves, the heart, lungs, kidneys and spleen, all working cooperatively—
that I have suggested a special designation for these states, homeostasis”
(Davies, 2016).
The homeostasis principle is then the property of a physiological system
to regulate its internal environment to a given set point in presence of a spe-
cific stimulus producing changes in that variable. The objective is to main-
tain stable and relatively constant physiological behavior. The control
process in the human body is ensured through the coordination of the con-
trol center and the natural sensors and effectors as shown in Fig. 10. The
control center is composed of the nervous system and the endocrine system.

3.2 Homeostasis examples

Fig. 11 introduces several examples of homeostasis, including energy and
fluid balances. A review on physiological energy homeostasis can be found
in Chapelot and Charlot (2019).
In the human body, the most typical example of homeostasis is the tem-
perature control process described in Fig. 12. In this example, the temperature
should be kept close to 37°C. When a stimulus occurs, the equilibrium is
restored either by sweating or by reducing blood circulation to the skin. Thus,
any change that either raises or lowers the set point temperature is

Fig. 10 Homeostasis principle.